Method and apparatus for casting metals

ABSTRACT

Method and apparatus for casting a metal article in a mold at least as long as the article, utilizing a cooled mold of elongated form having top and bottom portions. The method includes the steps of introducing molten metal from a source through the bottom portion of the mold, flowing molten metal into the mold so as to form a solidifying casting shell which .Iadd.a .Iaddend.during casting occupies at least 40% of the cross-sectional mold area and .Iadd.a .Iaddend.has a molten core, and flowing molten metal from the source through the core towards the mold top .Iadd.at a filling rate, dependent on the mold cross sectional area, such that the product produced will have a relatively fine grained structure throughout the cross-section thereof.Iaddend..

This invention relates to casting elongated metal articles such asbillets in a mold at least as long as the cast article. It provides forcontrol of the rate of solidification of the article during casting forimproved physical and morphological characteristics of the cast metalincluding, but not limited to, significantly improved surfacecharacteristics such as smoothness and subsurface inclusiondistribution, for example. It also provides an improved internalmicrostructure of the casting.

Heretofore, both in casting metal articles of greater or lesser lengththan their molds, i.e., continuous and non-continuous casting, attemptshave been made to control the rate of solidifying metal along asolid/liquid interface during casting, i.e., within the casting at theinterface of the solidifying casting shell and the molten metal. Theseattempts have been characterized by the use of costly andspace-consuming apparatus added to the basic casting apparatus to adjustflow of molten metal about the solid/liquid interface subsequent to andindependently of the introduction of molten metal into a mold. Suchapparatus has taken the form of molten metal stirrers and inductioncoils, either about the mold or the casting as in Tzavaras U.S. Pat. No.3,693,697. These coils generate moving magnetic fields in the moltenmetal which induce flow along such interface for the purpose of removingor inhibiting columnar dendrites, effecting improved dispersion ofchemical solutes and inhibiting stratification of inclusions within themajor portion of the casting, while inhibiting central porosity.

In my U.S. Pat. No. 3,517,725 issued June 30, 1970, there is disclosed atechnique of continuous casting of metal wherein billets of steel andother metals are cast from a closed-end, cooled mold which is relativelyseparated from a source of molten metal. The closed-end mold forms ashell of the billet being cast through which molten metal flows to therelatively retreating mold, the mold forming the outer shell of thebillet at its end remote from the source of molten metal.

In casting metals in a cooled or uncooled mold at least as long as thecasting, as opposed to the continuous casting technique of the aforesaidU.S. Pat. No. 3,517,725, it has been a practice to pour molten metalinto the mold through the bottom thereof to a height within the mold.During such pouring, a casting shell solidifies against the mold, andthe molten metal flows through the shell from the bottom to the top.However, the shell forms significantly less than 40% of the moldcross-sectional area during such casting during mold filling. While thevolumetric flow into such mold may be relatively large, there is littleor no washing effect of the relatively large liquid core on thesolid/liquid interface. Hence, unless otherwise controlled, the rate ofsolidification cannot be effectively governed, and solidification willtake place with the undesirable growth of columnar dendrites resultingin localized solute concentrations, segregated inclusions and centerlineporosity, all undesirable in a casting microstructure.

The present invention overcomes many of these difficulties with theprior art.

It is an object of the present invention to achieve the effect of suchsolidification control apparatus at least largely as a function ofmolten metal introduction into a mold, to dispense with such costly andcumbersome apparatus of addition, and to generally improve the techniqueof casting elongated metal articles, particularly those having a meltingpoint in the range of approximately 1,088° to 1,643° C. This inventionprovides a method and apparatus for casting a metal article in a mold atleast as long as the article, utilizing a cooled mold of elongated formhaving top and bottom portions. The method includes the steps ofintroducing molten metal from a source through the bottom portion of themold, flowing molten metal into the mold so as to form a solidifyingcasting shell which .Iadd.typically .Iaddend.during casting occupies atleast 40% of the cross-sectional mold area and has a molten core, andflowing molten metal from the source through the core towards the moldtop.

In the drawings

FIG. 1 is a broken, somewhat diagrammatic, side elevational viewpartially in median section, illustrating apparatus embodying theinvention, the apparatus being shown during a casting operation;

FIG. 2 is a sectional view taken on line 2--2 of FIG. 1; and

FIG. 3 is a graph showing molten metal introduction rates with referenceto molds of different dimensional cross sections.

In FIG. 1 of the drawing, a source of molten metal is indicatedgenerally at 10, which source may conveniently take the form of a ladlewhich, by way of example, may contain molten steel to be cast. Thesource is provided with a cover 12 secured in gas-tight relation to theladle, as by clamps 14, and has a gas inlet 16 therethrough connected toa pressurizing gas source 17, which may be nitrogen, for example. Thecover 12 suspends in source 10 a refractory pipe 18 having an inet endapproaching the bottom of the source 10. The interior of the pipe 18communicates with a passageway 22 extending through a refractory block20 supported by the cover 12. A gate valve 23, shown in the openposition thereof, places the passageway 22 in communication with arefractory nozzle 24. The nozzle extends into the lower end of a mold ofelongated form which, by way of example, may be of circularcross-section. The mold is indicated at 26 and is shown inclined to thevertical. In practice, the inclination may be approximately 86°, forexample. In principle, the mold may be vertical. However, thehydrostatic pressure of molten metal in such a vertically oriented moldwould give rise to such problems as to tend to make such orientationimpractical, such as the necessarily relatively large thickness of themold wall structure and the relatively high pressure required to forcemolten metal upwardly in such an oriented mold. Such a verticallyarranged mold would impart a relatively small meniscus to the moltenmetal therein, and such a small meniscus is desirable for reasons whichwill appear hereinafter. The apparatus would be inoperable if the moldwere horizontal so that the meniscus of the molten metal thereinextended from mold end to mold end as will appear. The upper end of mold26 is capped, as at 28, by a cap which has a vent hole therethrough forpassing gasses from the casting operation and which vent also indicateswhen the mold is filled by the appearance of metal therethrough. In FIG.1, the mold 26 is shown as having an intermedicate portion thereofmelted away, but it is to be understood that at the commencement of acasting operation, the mold is continuous from bottom to top.

In the form of the invention shown by way of example, the mold 26 is ofa sacrificial nature in that it melts away subsequent to the filling ofthe mold or a portion thereof with molten metal which molten metalsolidifies against the cooled wall of the mold prior to the melting offof the mold as will appear more fully hereinafter. Circumferentiallyarranged around the mold 26 in spaced relation are a plurality oflongitudinally extending pipe 27 for conducting a coolant from a sourcenot shown, such coolant, such as water, is jetted against the exteriorof the mold during a casting operation as will appear hereinafter, thevalves or jets being indicated at 30. The jets 30 may be controlled bythermocouples 32 placed against the side of the mold at axially spacedintervals. The thermocouples 32, which sense rising temperature in themold; are coupled to a nonillustrated spray or jet-controlling devicewhich may maintain impingement of the coolant against the lower end ofthe mold throughout a mold filling operation, and provides advancementof a cooling front along an axial portion of the mold in advance of andbehind the molten meniscus 33 within the mold during filling, whileterminating the impingement of the coolant against the mold a distance,indicated at 35, behind the advancing meniscus, so that in thelast-mentioned area the mold may melt off the casting shell. The mold 26is constructed of a metal characterized by high thermal conductivity inthe approximate range of 0.20 cal/cm² /cm/°C./sec. to 0.65 cal/cm²/cm/°C./sec. and having a melting point in the approximate range of 476°to 754° C. The mold may be structured to have a wall thickness in theapproximate range of 1.27 mm to 12.70 mm in a mold length ofapproximately 7.62 to 60.96 meters. Such a mold may be utilized to casta metal having a melting point in the approximate range of 1,088° to1,643° C., having a maximum thermal conductivity of approximately 0.25cal/cm² /cm/°C./sec. The mold which has been described is the presentlypreferred form only. Other types of bottom-filled molds may be suitablefor use with the casting technique described herein.

The mold 26 is supported by columnar supports 34 spaced axially alongthe mold as shown in FIGS. 1 and 2. The upper part of one such column 34is shown in FIG. 2 wherein the mold 26 has substantially only linecontact with the supporting structure to enable the coolant spray tocover the mold substantially in its entirety to prevent prematuremelting of the mold. Such support is provided by hollow, open-endedmetal block 36 of good thermal conductivity having a V-shaped recess 38therein which receives the mold 26 as shown, the block 36 being fixed tothe upper end of the column 34.

A tank, indicated generally at 40, is supported from the columns 34 toextend under the mold 26 to catch the coolant as it falls afterimpingement against the mold 26, and also to catch the molten metal fromthe mold 26 as the mold melts. The tank 40 has a suitable baffle 42 forcatching such coolant and metal from the mold. The tank 40 may beprovided with a suitable nonillustrated drain, and may be cleanedperiodically of the metal collected therein. The metal from the mold isin a molten state when falling and is solidified on contact with wateror in passing to the tank.

The apparatus of FIG. 1 is shown during a casting operation, i.e.,during the filling of the mold 26 and prior to completion of suchfilling. Prior to commencement of a casting operation and duringassembly of mold 26 with the casting nozzle 24, a quantity of a typicalcasting powder is inserted in the lower part of the mold, usually in aplastic container which will burn away during exposure to molten metal,to expose the powder therein to such molten metal. The powder 44liquifies where it interfaces with such metal but, because of its lesserdensity, rides on the metal meniscus, as shown. The action of suchpowder will appear hereinafter. On commencement of the castingoperation, the pressurizing gas is inletted through the inlet 16 intothe metal source 10 causing the molten metal to flow into the inlet endof the pipe 18 and through the block 20 and then open gate valve 23 tothe nozzle 24 and into the bottom of the mold 26. The filling rate ofthe mold is governed by the rising gas pressure in the source 10. As thefilling level in the mold advances, molten metal in contact with thecooled mold forms a solidifying shell, indicated at 46, which .Iadd.inthe examples described .Iaddend.occupies at least 40% of thecross-sectional area of the mold and has running therethrough a moltencore 48. The introduction of the molten metal into the mold is such thatthe liquid core 48 sweeps the solid/liquid interface formed in part bythe shell 46 in such manner as to inhibit columnar dendritic growthduring filling of the mold. It is believed that such sweeping action ofthe molten core results in at least partially equiaxed dendritic growthbut other microstructures are possible. Such sweeping action of theliquid core inhibits the formation of a so-called mushy zone between thetruly liquid core and the solidifying shell 46 to thereby enhance thedesired thermal gradient for proper solidification of the casting. Sucha thermal gradient exists both in longitudinal and transverse directionswith reference to the axis of the casting. The axial gradient is highestat the nozzle end of the casting. Solute elements tend to be disperseduniformly throughout the casting and a portion of inclusions tend to becaptured by the casting powder 44 as the liquid metal washes across suchcasting powder 44 on the filling of the mold. Such chemical and physicalaction during the casting operation is detailed in Tzavaras U.S. Pat.No. 3,693,697. The aforementioned inclusions, formed prior tosolidification, are mainly oxides which are stable at high temperatures.Inclusions formed during solidification are mostly sulfides, tellurides,arsenides, nitrides and some oxides. The usual inclusions in steel arecompounds of various solutes or deoxidizers used in steel combined withoxygen, sulphur, and less frequently with nitrogen. The aforementionedsolutes, e.g., in steel, are elements other than iron, such as alloyingelements. Such pressure pouring of the casting is completed on fillingthe mold 26 to the top. On completion of the mold filling, the gatevalve 23 is closed and the remaining cooling jets are turned off. Whenall the coolant jets are turned off, the remaining portion of the moldrises in temperature and is melted off leaving the casting supported inthe manner in which the mold was previously supported from the columns34. However, the portion of the mold 26 in which the cap 28 is insertedmay not melt off. Such a cast metal product exhibits superior surfacecharacteristics, among others. If desired, several such molds may befilled simultaneously from a single molten metal source as will beobvious. While pressure pouring of the molten metal has been describedwith reference to the casting technique, it will be evident to thoseskilled in the art that the mold 26, may be filled by vacuum pouring,and indeed the mold may be filled from the bottom by other pouringtechniques.

EXAMPLE

The metal to be cast is AISI-304 stainless steel having a composition of0.08% carbon max., 2.0% Mn max., 1.0% Si max., 18-20% Cr, 8.0-11% Ni,0.040% P max., 0.030% S max., (balance Fe) and solidus at 1,427° C. andliquidus at 1,510° C., wherein the thermal conductivity is 0.039 cal/cm²/cm/°C./se at 100° C. The mold composition is aluminum 2024 alloy havinga composition of 4.5% Cu, 1.5% Mg, 6% Mn (Balance Al) and solidus at502° C. and liquidus at 638° C., wherein the thermal conductivity is0.45 cal/cm² /cm/°C./sec. at 25° C. The mold is 30.48 meters long, has awall thickness of 4.75 mm and has an internal cross-section of 100mm×100 mm. The mold filling rate by pressure pouring is 4.834 kg/cm²/min (483.4 kg/min). The liquid core diameter is approximately 50.8 mmwhile the liquid core velocity is 31.39 meters/min approximately.

There is shown in FIG. 3 a graph illustrating different metalintroduction rates with reference to molds of different cross-sectionalareas. As shown there, a mold having a cross-sectional area of 25.81 cm²has molten metal introduced thereinto for filling at a rate betweenapproximately 2.39-23.9 kg/cm² /min. There is also shown that for a moldhaving a cross-sectional area of 967.74 cm² the molten metalintroduction rate is between approximately 0.239-4.78 kg/cm² /min. inthis non-linear relationship. This relationship requires the use of amold whose thermal conductivity is at least 0.20 cal/cm² /cm/°C./sec.Molds having intermediate cross-sectional areas have intermediate moltenmetal introduction rates as indicated by the graph.

While several forms of the method and apparatus for casting a metalarticle have been described, it will be apparent, especially to thoseversed in the art, that the invention may take other forms and issusceptible to various changes in details without departing from theprinciples of the invention.

What is claimed is:
 1. A method of casting a metal article in anelongated mold having a cross-sectional area substantially between 25.8cm² and 967.74 cm², .[.a thermal conductivity of at least 0.20 cal/cm²/cm°C./sec.,.]. and a length at least as long as the article to be cast,said mold having top and bottom portions comprising:inclining said moldto the vertical; introducing molten metal from a source through saidbottom portion of said mold at a filling rate substantially between0.239 kg/cm² /min. and 23.9 kg/cm² /min., the combination of fillingrate and cross-sectional area of the mold being defined by the hatchedarea of the graph as shown in FIG. 3; flowing molten metal along saidmold by applying a differential pressure across said source and said topportion of said mold; cooling the mold while regulating the flow of saidmolten metal therealong to form a solidified casting shell which.[.,during casting, occupies at least 40% of the cross-sectional mold areaand.]. has a molten core; flowing molten metal from said source toextend said casting shell and said molten core toward said top portionof said mold; and subsequently solidifying said molten core .Iadd.toproduce a product having a relatively fine-grained structure throughoutthe cross-section thereof.Iaddend..
 2. A method as defined in claim 1wherein: said metal of said mold has a thermal conductivity of about0.20 cal/cm² /cm/°C./sec. to 0.65 cal/cm² /cm/°C./sec.
 3. A method asdefined in claim 1, further including placing a casting powder in saidmold for exposure to said molten metal.
 4. A method as defined in claim1, wherein: the filling rate of a mold cross section of 25.81 cm² isbetween approximately 2.39-23.9 kg/cm² /min. and of a mold cross sectionof 967.74 cm² is between approximately 0.239-4.78 kg/cm² /min. and for amold cross section therebetween has an intermediate filling rate.
 5. Amethod as defined in claim 1, wherein: said mold is structured of metalof relatively high thermal conductivity and having a relatively thinwall, and wherein said cooling step comprises directing jets of coolanton at least a part of said wall, and discontinuing directing said jetsover a portion of said wall during flow of molten metal through saidcore to melt off at least said portion of said wall from said article.6. A method as defined in claim 5, wherein: said metal of said mold hasa melting point of about 476° to 754° C., said metal of said castarticle having a melting point of about 1,088° C. to 1,643° C.
 7. Amethod as defined in claim 5, further including supporting the mold fromat least one support element, and supporting the casting from saidsupport element on the melting off of at least a portion of said moldfrom said article.
 8. A method as defined in claim 5, wherein: said moldwall thickness is about 1.27 to 12.7 mm.
 9. Apparatus for casting anelongated article comprising:a source of molten metal; a mold inclinedtowards the vertical and being at least as long as said article to becast, said mold associated with said source of molten metal having abottom including an opening and a top portion, said mold further havinga cross-sectional area between 25.8 cm² and 967.74 cm² .[.and a thermalconductivity of at least 0.20 cal/cm² /cm/°C./sec..].; means forintroducing molten metal into said mold opening at a filling ratesubstantially between 0.239 kg/cm² /min. and 23.9 kg/cm² /min., thecombination of filling rate and cross-sectional area of the mold beingdefined by the hatched area of the graph as shown in FIG. 3, saidintroducing means associated with said source of molten metal includingmeans for applying a differential pressure between said source and saidtop portion of said mold to regulate the flow of molten metal along saidmold; means cooling said mold to form a solidifying casting shellwhich.[., during casting, occupies at least 40% of the cross-sectionalmold area and.]. has a molten core, said introducing and meansassociated with said mold for flowing molten metal from said sourcealong said core to extend said casting shell and said molten core towardsaid top of said mold and after casting is completed for solidifyingsaid molten core .Iadd.to produce a product having a relativelyfine-grained structure throughout the cross-section thereof.Iaddend..10. A mold as defined in claim 9, wherein: said mold is constructed ofmetal of relatively high thermal conductivity and having a relativelythin wall, said cooling means comprising means for directing coolantjets over at least part of said wall, and further including valve meansassociated with said cooling means for shutting down at least some ofsaid jets during filling of said mold, so as to raise the temperature ofand melt off at least a portion of said mold, said metal of said moldhaving a melting point of about 476° to 754° C. and said metal of saidcast article having a melting point of about 1,088° to 1,643° C. 11.Apparatus as defined in claim 10, wherein: said metal of said mold has athermal conductivity of about 0.20 cal/cm² /cm/°C./sec. to 0.65 cal/cm²/cm/°C./sec.
 12. Apparatus as defined in claim 11, wherein: said moldwall thickness is about 1.27 mm to 12.70 mm.